How many moderated user testings can a researcher handle?

I enjoy doing user testing as a moderator, i.e. usability testing, contextual inquiry, but it's also emotionally draining for me (I wonder why?) Currently I'm doing it only once in a while. Majority of my time is doing other non-customer facing researches. But I am considering increasing it. Or even doing this full time.

My question to my fellow user researcher is, if this is your main task, how many can you handle, say, per week, per month, without it being too exhausting for you?

Thanks,